I don't want to put off anyone building a 90 bevel gearbox, but it is tough work. You need to have a way of stabilizing the gears (usually bearings), then you need a pillowblock for the bearings - onto of that, you need to make everything small enough that it is about 1.5 inchs length and width. I did it, and it wasn't worth it after seeing the purchased ones (gear boxes) which are of better strength and alot less time. But if you want to tinker, tinker away, just the product of my experience with trying to build one of these darn thing cheaply, in the end, it wasn't worth it to me.

Oh, by the way, the chuck doesn't have the correct pitch for use in a bevel 90 degree gearing system.

hey , about the gear ratio, you do not need to worry if you think twice.
The size of gear between the knob and the gear on the lens is almost irrelevant, if the gear on the lens is same diameter as the knob. All the rest in between is just direct drive with no speed reduction.

Giroud, that's actually incorrect. The size of the bevel gears, if they are 1:1, is irrelevant. The drive gear (the smaller one that drives the gear on the lens) absolutely affects the ratio of how much knob rotation equates to lens rotation. The diameter of the knob only affects how much effort you have to exert.

I don't think so.
if you have only gear that act by there external diameter on all the transmission line (how it must be the case on an amateur project) you can not have any reduction.
To have a reduction, you need a double gear (one that has teeth close to the axis and teeth on the external diameter), but in that case only the speed change will require a proportional force change too (energy is conserved)
If you have simple gear all touching only by external diameter and both ends finish with approximately the same diameter, the ratio will be anyway close to 1:1

imagine it like this. take the diameter of the drive gear (the gear that touches with the lens gear) and take the diameter of the lens gear.
NOW, turn the drive one full rotation. (covering the entire diatmer of the drive gear). Is the lens gear going to turn gear one full rotation? no. Only the Knob and driver gear will turn one full rotation. the lens gear will only turn as much as the diameter of the small drive gear is equal to. The size of the knob has NOTHING to do with it. If you turn the knob one full turn, the drive gear does one full turn also with it. But the lense gear (being much larger) will only do a fraction of that turn because of gear reduction.
lets say one full turn of the drive gear causes all 40 teeth to interlock during the process. That means that only 40 teeth of the lens gear will interlock also. But the lens gear might have lets say 100 teeth. That means the drive gear cannot be in a 1:1 ratio with the lens gear. Unless you build some sort of gearbox to correct the difference.

Ok so you just need to mount the knob on the same gear that the lens and the axis that will drive these two gear should have the same small gear at bothe end. Seems easy, but i am not sur that is usefull since you could loose some precision in focusing. (you rarely need to focus from one extrem of the lens to the other, but more often need to slightly adjust on a small range.)

Giroud, I am not sure what you are trying to achieve but the goal of the usual person's follow focus is to allow them good fine-tuning focus adjustment via a reasonable number of knob rotations, without high manual effort.

The way to achieve this is by using a large enough knob, and a low enough gear reduction at the lens.

I tried the helicopter gear but unfortunately it wasn't the right pitch. It still rotates the focus ring but it rattles fairly loud. I strapped a rubberband around it and that helps but I still think it can be better. Does anyone know the pitch for the cavision follow focus? Or is there a way I can measure it?

I am not sure if people know that when I speak of PITCH I am referring to the gears in the 90 degree gear box. Pitch does not refer to the lens gear or the gear that touches the lens. The Pitch is the angle that one gear meshes with the other gear. I don't know if that is clear or not...

I have noticed there is considerable confusion/discrepancy in the ways people discuss gearing. If you have any insights, people would love to hear them. Would you be interested in posting a tutorial? You wouldn't even have to show your follow-focus unit, just indicate how a gear with 12 notches can mate with one of, say 1200. (I don't even know the terms of discussion here, other than pitch.) And what those pertinent dimensions are in terms of pitch, size or other. If this is information already available (and it probably is), perhaps you could point us to it with a few notes.